Radiosurgery and radiotherapy systems are radiation treatment systems that use external radiation beams to treat pathological anatomies (e.g., tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering a prescribed dose of radiation (e.g., X-rays or gamma rays) to the pathological anatomy while minimizing radiation exposure to surrounding tissue and critical anatomical structures (e.g., the spinal chord). Both radiosurgery and radiotherapy are designed to necrotize the pathological anatomy while sparing healthy tissue and the critical structures. Radiotherapy is characterized by a low radiation dose per treatment, and many treatments (e.g., 30 to 45 days of treatment). Radiosurgery is characterized by a relatively high radiation dose in one, or at most a few, treatments.
In both radiotherapy and radiosurgery, the radiation dose is delivered to the site of the pathological anatomy from multiple angles. As the angle of each radiation beam is different, each beam can intersect a target region occupied by the pathological anatomy, while passing through different regions of healthy tissue on its way to and from the target region. As a result, the cumulative radiation dose in the target region is high and the average radiation dose to healthy tissue and critical structures is low. Radiotherapy and radiosurgery treatment systems can be classified as frame-based or image-guided.
In frame-based radiosurgery and radiotherapy, a rigid and invasive frame is fixed to the patient to immobilize the patient throughout a diagnostic imaging and treatment planning phase, and a subsequent treatment delivery phase. The frame is fixed on the patient during the entire process. Image-guided radiosurgery and radiotherapy (IGR) eliminate the need for invasive frame fixation by tracking and correcting for patient movement during treatment.
Image-guided radiotherapy and radiosurgery systems include gantry-based systems and robotic-based systems. In gantry-based systems, the radiation source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. Each time a radiation beam is delivered during treatment, the axis of the beam passes through the isocenter. In some gantry-based systems, known as intensity modulated radiation therapy (IMRT) systems, the cross-section of the beam is shaped to conform the beam to the pathological anatomy under treatment. In robotic-based systems, the radiation source is not constrained to a single plane of rotation. -
In image-guided systems, patient tracking during treatment is accomplished by registering two-dimensional (2-D) in-treatment X-ray images of the patient (indicating where the patient is) to 2-D reference projections of one or more pre-treatment three-dimensional (3-D) volume studies of the patient (indicating where the patient should be to match the treatment plan). The pre-treatment 3-D volume studies may be computed tomography (CT) scans, magnetic resonance imaging (MRI) scans, positron emission tomography (PET) scans or the like.
The reference projections (reference images), known as digitally reconstructed radiographs (DRRs) are generated using ray-tracing algorithms that replicate the geometry of the in-treatment X-ray imaging system to produce images that have the same scale as the in-treatment X-ray images. Typically, the in-treatment X-ray system is stereoscopic, producing images of the patient from two different points of view (e.g., orthogonal views).
The registration process compares the in-treatment X-ray images and the DRRs in each projection, and produces independent difference measures in each projection, which are sensitive to registration parameters such as rotational and translational misalignments. Each projection is characterized by its own search space where the registration parameters can be searched to minimize the associated difference measure for that projection. However, two of the registration parameters in one projection are coupled to the registration parameters in the other projection by the geometry of the X-ray imaging system. As a result, these two common registration parameters that are independently estimated in each projection may be not consistent . . . . Therefore, the registration process alternates between the two projections to maximize the overall registration, and the overall registration process may be slowed.